a) Field of the Invention
The invention is directed to an arrangement for debris reduction in a radiation source based on a plasma, particularly for generating bundled radiation in the extreme ultraviolet (EUV) spectral region. The invention is preferably applied in semiconductor lithography for exposure of very small structures on semiconductor chips.
b) Description of the Related Art
Sources of extreme ultraviolet (EUV) radiation or soft x-ray radiation are urgently needed as radiation sources for the next generation of exposure machines in semiconductor lithography. The structure widths of integrated circuits should be reduced to a width between 25 nm and 50 nm with radiation at around 13.5 nm. Since the radiation for the desired wavelength range is generated from a hot plasma, an EUV radiation source also emits, in addition to the actual radiation, neutral particles and electrically charged particles with kinetic energies in the keV range. The totality of this particle emission is called debris. The debris causes reduced reflectivity of the collector optics arranged downstream of the source location (plasma) by wearing away the reflecting layer (substantial effect) or by coating this layer with particles which absorb the EUV radiation. The debris limits the life of the collector optics. The life of the collector optics is stated as a quantity of EUV radiation pulses after which the reflectivity of the collector optics drops by 10%. The producers of scanners for the semiconductor lithography process require an optics life of greater than 1011 shots (radiation pulses).
Various debris filters, known as debris mitigation tools (DMT), are known for reducing debris and thus protecting the collector optics from rapid aging through the processes mentioned above. For example, U.S. Pat. No. 6,359,969 describes various honeycombed arrangements of metal plates which can have various suitable shapes. Gases (e.g., argon) flow at sufficient pressure into the individual honeycombs. The gas causes a sharp deceleration of the debris particles until their kinetic energy is less than kT and the particles are subsequently easily adsorbed at the honeycomb structure.
In various other arrangements (e.g., DE 102 15 469.4 or U.S. Pat. No. 6,377,651), electric and/or magnetic fields are used to deflect charged particles.
Further, as is disclosed, e.g., in Yamamoto et al., Proc. of SPIE, Vol. 4146 (2000) 128, fast shutters are known which quickly close the radiation path after every radiation pulse and accordingly block slower particles. However, these shutters fail to function at the required pulse repetition frequencies of several kilohertz.
In other suggestions, a debris filter such as that also known from U.S. Pat. No. 6,359,969 is combined with EUV-transparent foils to achieve a complete filtering of debris. However, there is a considerable risk that these foils will be destroyed by especially high-energy particles. Particularly with EUV sources with high average outputs, there is a very high probability that the foil will be destroyed due to the inability to estimate for preventative purposes, which would mean a sudden and rapid deterioration of imaging conditions.